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SEPTEMBER 2018 S H I M A D A A N D H O R I N O U C H I 2799 Reintensification and Eyewall Formation in Strong Shear: A Case Study of Typhoon Noul (2015) UDAI SHIMADA Meteorological Research Institute, Tsukuba, Ibaraki, Japan TAKESHI HORINOUCHI Faculty of Environmental Earth Science, Hokkaido University, Sapporo, Hokkaido, Japan (Manuscript received 27 January 2018, in final form 9 July 2018) ABSTRACT Strong vertical wind shear produces asymmetries in the eyewall structure of a tropical cyclone (TC) and is generally a hostile environment for TC intensification. Typhoon Noul (2015), however, reintensified and 2 formed a closed eyewall despite 200–850-hPa vertical shear in excess of 11 m s 1. Noul’s reintensification and eyewall formation in strong shear were examined by using Doppler radar and surface observations. The evolution of the azimuthal-mean structure showed that the tangential wind at 2-km altitude increased from 30 2 to 45 m s 1 in only 5 h. During the first half of the reintensification, the azimuthal-mean inflow penetrated into the ;40-km radius, well inside the radius of maximum wind (RMW), at least below 4-km altitude, and re- flectivity inside the RMW increased. As for the asymmetric evolution, vigorous convection, dominated by an azimuthal wavenumber-1 asymmetry, occurred in the downshear-left quadrant when shear started to increase and then moved upshear. A mesovortex formed inside the convective asymmetry on the upshear side. The direction of vortex tilt between the 1- and 5-km altitudes rotated cyclonically from the downshear-left to the upshear-right quadrant as the vortex was vertically aligned. In conjunction with the alignment, the amplitude of the wavenumber-1 convective asymmetry decreased and a closed eyewall formed. These features are consistent with the theory that a vortex can be vertically aligned through upshear precession. The analysis results suggest that the vortex tilt, vigorous convection, and subsequent intensification were triggered by the increase in shear in a convectively favorable environment. 1. Introduction weakening. Explanations for the negative influence of shear on intensity include the following: (i) ventilation Tropical cyclones (TCs) form, intensify, and weaken of the TC warm core and high potential vorticity (PV) in the presence of vertical wind shear, and shear is one of by a relatively strong upper-level environmental wind the most important factors accounting for changes in (Frank and Ritchie 2001); (ii) strong downdrafts bring- the structure and intensity of a TC. The interaction ing midlevel low equivalent potential temperature (u ) processes between a TC and shear, however, are very e air into the boundary layer, leading to a decrease in TC complicated and difficult to predict. intensity from the perspective of a Carnot cycle heat Vertical shear is known to be unfavorable for in- engine (Riemer et al. 2010); and (iii) intrusion of mid- tensification. Statistically, intensifying TCs have weaker level dry air into the eyewall (Tang and Emanuel 2010). shear than nonintensifying TCs (Kaplan and DeMaria In addition, the effect of shear on intensity change is 2003). Further, shear predictors in statistical dynamical dependent not only on the shear magnitude, but also on models have a weakening effect on intensity (DeMaria vortex strength: stronger vortices are more resilient to and Kaplan 1994, 1999; Knaff et al. 2005; DeMaria 2009; shear (DeMaria 1996; Riemer et al. 2013; Rios-Berrios Kaplan et al. 2010). Paterson et al. (2005) and Wang and Torn 2017). et al. (2015) showed statistically that a 200–850-hPa 2 Jones (1995) and Reasor et al. (2004) theorized that wind shear of greater than ;10 m s 1 contributes to such TC vortices have intrinsic resiliency against shear that keeps them vertically aligned. A tilted PV column can Corresponding author: Udai Shimada, [email protected] be decomposed into azimuthal-mean and wavenumber-1 DOI: 10.1175/MWR-D-18-0035.1 Ó 2018 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses). Unauthenticated | Downloaded 10/04/21 11:42 AM UTC 2800 MONTHLY WEATHER REVIEW VOLUME 146 components (Reasor and Montgomery 2001). When the initiated by low-level deep inflow in the downshear-right scale of the vortex is much smaller than the Rossby ra- quadrant. Intense updrafts grow hydrometeors down- dius of deformation, the wavenumber-1 asymmetry be- shear, causing radar reflectivity to be highest in the haves like a vortex Rossby wave (VRW) called a tilt downshear-left quadrant. Downdrafts, which are asso- mode (Reasor et al. 2004), which is similar to an edge ciated with small-scale deep convection downshear left, wave on a Rankine vortex (Lamb 1932). The vortex dominate the upper levels of the upshear-left quadrant. precesses as the VRW propagates (Jones 1995) and is Eyewall convection on the upshear side is suppressed vertically aligned by the resonant damping of the tilt by upper-level inflow and low-level outflow. mode, provided that the sign of the radial gradient of Some studies, however, have shown that moderate PV at the critical radius, where the phase speed of the and strong shear2 can contribute to intensification VRW and tangential wind speed are the same, is nega- through the occurrence of vigorous deep convection, tive (Schecter et al. 2002; Schecter and Montgomery called convective bursts, inside the radius of maximum 1 2003). In this case, the vortex reaches an equilibrium tilt wind (RMW) on the downshear side (Molinari et al. of 908 to the left of shear (Reasor et al. 2004). In contrast, 2006; Reasor et al. 2009; Molinari and Vollaro 2010; the vortex shears apart when the gradient at the critical Nguyen and Molinari 2012). Molinari and Vollaro (2010) radius is positive (Schecter et al. 2002). Meanwhile, examined the intensification (a pressure fall of 22 hPa in when the PV gradient is 0 at the critical radius, the less than 3 h) of Tropical Storm Gabrielle (2001) in the 2 vortex undergoes repetitive cycles of tilting downshear, presence of a shear of 13 m s 1, and Nguyen and cyclonic precession upshear, and realignment (Reasor Molinari (2012) examined the intensification (an in- et al. 2004; Reasor and Montgomery 2015). Reasor et al. crease in the maximum 1-min sustained wind speed from 2 (2013) showed through a composite analysis of obser- ;33.4 to 48.9 m s 1 in 18 h) of Hurricane Irene (1999) vations that TC vortices tend to be tilted downshear and in an environment of increasing shear, from 6–7 to 2 to the left of the shear vector. 10–13 m s 1. Both Molinari and Vollaro (2010) and Previous studies have shown that maximum upward Nguyen and Molinari (2012) discussed that these storms motion occurs in the downtilt direction, rather than in were highly asymmetric, but that it was the projection of the downshear direction (Rogers et al. 2003; Zhu et al. the asymmetric heating onto the axisymmetric compo- 2004; Wu et al. 2006; Braun et al. 2006; Braun and Wu nent that was critical for intensification, based on the 2007; Ueno 2008; Riemer et al. 2010; Reasor and Eastin theoretical studies of Nolan and Grasso (2003), and 2012; Reasor et al. 2013), and mechanisms have been Nolan et al. (2007). Meanwhile, Reasor et al. (2009) proposed for this vertical motion asymmetry. When a showed that Hurricane Guillermo (1997) intensified vortex is tilted by shear, adiabatic upward (downward) rapidly with asymmetric strong convection in the motion is induced downtilt (uptilt) as a transient re- 2 downshear-left quadrant under a shear of ;7–8 m s 1 sponse to maintain thermal wind balance in the tilted and emphasized the nonnegligible contribution of eddy vortex, and this response causes a negative (positive) momentum fluxes. temperature anomaly downtilt (uptilt) (Jones 1995; Wang Even under the same moderate-shear environment, and Holland 1996; Frank and Ritchie 1999; Ueno 2007). both intensifying and nonintensifying TCs are observed. Following this response, diabatic processes allow a Rogers et al. (2016) examined structural changes of balance to be established between horizontal advec- Hurricane Edouard (2014) and showed that Edouard tion of vorticity by low-level storm-relative inflow had deep vigorous convection in the upshear-left (outflow) and vortex stretching (shrinking) on the low- quadrant during intensification but little deep convec- level inflow (outflow) side (Bender 1997; Frank and tion there during its maturity. Rios-Berrios and Torn Ritchie 1999, 2001). (2017) demonstrated, through a composite analysis of Through the mechanisms described above, vertical shear induces TC precipitation and convection structure intensifying and nonintensifying TCs under moderate with wavenumber-1 asymmetry (e.g., Black et al. 2002; shear conditions, that midlevel relative humidity and Corbosiero and Molinari 2002, 2003; Corbosiero et al. surface latent heat fluxes were larger in intensifying 2006; Chen et al. 2006). Observation-based composite TCs, in particular, on the upshear side, than in studies (Reasor et al. 2013; DeHart et al. 2014) have shown nonintensifying TCs. We lack sufficient knowledge, that for hurricane-strength TCs, eyewall convection is 2 Here, we use three categories of 200–850-hPa shear based on the 25th and 75th percentiles of shear magnitude distributions 2 1 Tilt is defined as the difference between the upper- and the around TCs (Rios-Berrios and Torn 2017): weak, ,4.5 m s 1; 2 2 lower-level vortex center locations. moderate, 4.5–11.0 m s 1; and strong, .11.0 m s 1. Unauthenticated | Downloaded 10/04/21 11:42 AM UTC SEPTEMBER 2018 S H I M A D A A N D H O R I N O U C H I 2801 FIG.